**Core Concept**
Aspirin irreversibly inhibits cyclooxygenase-1 (COX-1) in platelets, reducing the synthesis of thromboxane A2 (TXA2), a potent platelet aggregator and vasoconstrictor. This action prevents platelet activation and aggregation, which is central to aspirin’s anti-thrombotic effect in cardiovascular disease.

**Why the Correct Answer is Right**
Thromboxane A2 is produced by platelets via COX-1 and promotes platelet aggregation and vasoconstriction. Aspirin acetylates COX-1, blocking the enzyme’s ability to convert arachidonic acid into thromboxane A2. Since platelets lack the ability to synthesize new COX-1, this inhibition is long-lasting and effectively suppresses platelet aggregation. This mechanism is key in preventing thrombotic events like myocardial infarction and stroke.

**Why Each Wrong Option is Incorrect**
Option A: Prostacyclin is produced by endothelial cells and promotes vasodilation and inhibits platelet aggregation. Aspirin does not directly block prostacyclin; in fact, it may indirectly reduce it, but this is not the primary mechanism of anti-aggregation.
Option B: PGF₂α is involved in uterine contraction and inflammation, not platelet aggregation. It is not a major mediator in platelet function.
Option D: Phospholipase C is involved in intracellular signaling (e.g., in GPCR pathways), but it is not the direct target of aspirin. Aspirin acts on COX-1, not phospholipase C.

**Clinical Pearl / High-Yield Fact**
Aspirin's antiplatelet effect is due to irreversible inhibition of thromboxane A2 synthesis in platelets—this effect lasts for the lifespan of the platelet (7–10 days). This makes aspirin a cornerstone in secondary prevention of cardiovascular disease.

✓ Correct Answer: C. Thromboxane A2